Composite structural joint and method of fabrication thereof

ABSTRACT

A composite structural joint is reinforced and assembled utilizing barbed quill fasteners. By using such fasteners, the structural joint can be formed while in a staged condition. This allows co-curing and bonding of the elements of the structure in a joined condition. Additionally, by virtue of the use of the barbed quills and their being strategically implanted in the structural joint, peel forces in the joint are minimized.

BACKGROUND OF THE INVENTION

The present invention generally relates to a reinforced compositestructural joint. More particularly, the invention is directed to acomposite structural joint which is reinforced and assembled utilizingbarbed quills. The quills, by holding together the formed details, allowassembly of the composite structure while in a staged condition suchthat the entire structure can be co-cured in one operation. By virtue ofthe implanted quills, the finally formed composite structural joint issubstantially strengthened as a result of the peel forces in the jointbeing minimized.

Usage of composite materials in structures has been increasing rapidly.This is particularly true in the aerospace industry where thelightweight, high strength characteristics of these materials areparticularly desirable. In this art, strong, lightweight, tough,self-sustaining sheet material has been developed, these materials beingcomposed as a class, of a resinous sheet reinforced with layers ofcontinuous, lineally aligned, parallel filaments. These sheets can beformed as a single layer sheet or as multi-layer laminates, andthereafter thermoset to tough, hard, exceptionally strong panels, orskins for aircraft and the like.

As initially formed, these sheet materials are flexible and deformable,providing panel-forming members which can be draped or otherwiseconformed to various shapes and thereafter cured, by thermosetting, uponthe application of heat and pressure thereto, to tough, strong skins orpanels of permanent shape retention and having exceptional tensilestrength imparted by the continuous filament reinforcing.

Panels or skins made up of these materials are exceptionally strong,lightweight structural units. However, structural joints made by theinclusion of reinforcing ribs or the like on such panels have heretoforeseen limited applicability. Presently, these skins or panels aretypically joined by first curing the sheet material into the desiredshape and adhesively bonding a web or rib structure to the panel throughthe medium of a thermosetting adhesive.

However, as previously stated, uncured composite structure is flexibleand deformable. When curing large combined structural members, such asan aircraft wing box, difficulties have resulted in holding the parts inplace, especially when transporting an uncured structure to an autoclaveor oven. Further, it has been found that tension loads, at unacceptablylow values, on the cured panel or skin of the composite structure resultin "peel" forces which tend to separate the web and panel of thestructure at their interface.

PRIOR ART STATEMENT

Due to the formable nature of uncured composite laminated sheets, thecurrent practice has been to individually cure the elements of thestructure prior to assembling the structure. As such, the structure isnot assembled until the individual parts are cured. A further bonding orfastening operation is necessary once the structure is assembled.Various attempts have been made to solve the peeling problem in bondedjoints under tension load. Thus, metal rivets and bolts have been usedas fasteners at the interface of the composite rib/sheet joints.unfortunately, this has increased the weight of the resulting structure,increased fabrication time, required more plies of material due to thereduced laminate strength resulting from the necessary drilled fastenerholes, and presented sealing problems where the panel forms part of acontainer, such as a fuel tank. Imbedding a metal "T" element into ajoint has also been utilized. This has also been found to beunsatisfactory in that the element is expensive, increases the weight ofthe structure, requires a critical priming operation, is incompatiblewith the surrounding material due to its higher thermal co-efficient ofexpansion, and is too rigid, thereby resulting in a notch effect.

Applicant has found that the barbed quills according to the presentinvention can be easily inserted into the staged structure with minimaldisturbance to the fibers and hold together the elements of the stagedstructure because of the barbs becoming anchored in the compositefibers. With bolts or rivets, holes must be drilled in the elements ofthe structure which cuts the composite fibers, and are subject to theabove noted disadvantages. Further, there is normally no access toinstall corresponding fastener nuts (with of course the additionalconfiguration problems due to protrusion of the nut). Moreover, thecomposite elements are too soft, even in the staged condition, forefficient hole drilling and for the bolts to effectively grip thematerial. It has also been found that quills according to the presentinvention are cheaper and lighter than bolts or rivets, can be implantedinto the structure much quicker, do not waste material in that no holesare required, and result in only minimal disturbance to the compositefibers. In addition, when strategically placed, the barbed quills addstrength to the structure. This is accomplished by the elimination ofneed for holes and the avoidance of tearing of the fibers.

SUMMARY OF THE INVENTION

It is therefore, an object of the present invention, to provide areinforced composite structural joint with increased resistance to peelforces at the joint interface.

It is another object of the present invention to provide an efficientlyfastened staged composite structure with unimpaired structural strength.

It is still another object of the present invention to provide a methodof fabrication which results in a reinforced composite structure andallows co-curing of the assembly.

Briefly, in accordance with the invention, there is provided areinforced composite structure comprising a staged composite panel, astaged composite web, and a plurality of quills infixed in the panel andweb. The quills have barbs thereon which engage the fibers of the paneland the web. The quills join the panel to the web such that thestructure can be co-cured as assembled. Optimally, the quills arepositioned such that the structure is reinforced when cured. Optionally,a composite reinforcement member in the form of a generally wedge-shapedlaminate is provided. In this case, the reinforcement member would bebonded to the panel and web and also have the quills infixed therein.

In another form of the invention, there is provided a compositestructure comprising a composite panel, a composite web, a compositereinforcement member, and a plurality of barbed quills infixed in thepanel, the reinforcement member, and the web.

In still another form of the present invention, a method is providedwhich comprises staging of the composite panel and web, positioning thepanel relative to the web, infixing a plurality of quills in said paneland said web while they are in a staged condition such that the web andpanel are joined, and co-curing the web and panel as joined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a composite structural joint accordingto the present invention having headed barbed quill fasteners implantedtherein;

FIG. 2 is a perspective view of a barbed quill according to the presentinvention;

FIG. 3 is a perspective view of another form of barbed quill;

FIG. 4 is a perspective view of a composite structure illustratingheaded barbed quills as used in both assembly and strengthening of thestructural joints;

FIG. 5 is a perspective view of a reinforcement member; and

FIG. 6 is a perspective view of a reinforced composite structureillustrating a joint strengthened by a reinforcement member incombination with headed barbed quill fasteners.

While the invention will be described in connection with the preferredembodiments, it will be understood that it is not intended to limit theinvention to those embodiments. On the contrary, it is intended to coverall alternatives, modifications, and equivalents that may be includedwithin the spirit and scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is shown according to the presentinvention a reinforced composite structure generally indicated at 10.Structure 10 utilizes headed barbed quills 12 and 14, which are morefully described with reference to FIGS. 2 and 3, to reinforce thestructural joint which is defined by a composite web generally indicatedat 16 and composite panel or skin 18. Both web 16 and panel 18 arenormally laminates of a plurality of fiber composite plies. Web 16 ismade up of opposed composite sheets 20 and 22. A portion 24 of each ofsheets 20 and 22 is positioned parallel and adjacent to panel 18. Asecond portion 26 of sheets 20 and 22 extends transversely to panel 18.In the embodiment illustrated in FIG. 1, the second portion 26 isapproximately orthogonally disposed to panel 18. Sheets 20 and 24 alsohave a fillet portion 28 between the first portion 24 and second portion26. Positioned between sheets 20 and 22 of web 16 and panel 18 is acomposite filler material 30. A thermosetting adhesive is placed on thecontacting surfaces of web 16, panel 18, and filler 30, but is notsufficient to hold these members in assembled condition at roomtemperature prior to curing.

In the uncured condition, the composite sheet materials are flexible anddeformable. After conforming them to the desired shape, the compositemembers are, according to the present invention, staged which gives thema stiffness at room temperature which allows the members to be assembledand fastened to the desired form utilizing the quills herein described.Staging is accomplished by heating to a temperature normally less thanthe curing temperature and compressing the composite members.Specifically, it has been found that graphite epoxy laminates can beheated to a temperature of about 175° F for about 30 minutes to 1 hourwhile simultaneously compressed to cause bleeding (getting rid of excessresin) to result in a member which is suitably staged for the presentinvention.

In this staged condition, the composite members will still normally betoo soft to allow the installation of bolts in view of the difficulty ofhole drilling. In any case, as previously mentioned, hole drilling istime consuming, wastes material, results in fuel sealing problems, cutsthe composite fibers, requires the use of extra plies, and does notovercome the problem of access to apply the corresponding nuts andbolts.

FIGS. 2 and 3 illustrate barbed quills according to the presentinvention. In FIG. 2, the quill generally indicated by 33 has a head 41and a rod portion 35 preferably perpendicular to head 41. Rod 35 ispreferably tapered to allow easy insertion into the staged compositepanel and web. The distal end 49 of rod 35 is pointed. By virtue of thepointed distal end 49 and taper of rod 35, the fibers of the compositelaminates are spread rather than torn on insertion therein of thequills. As a result, structural strength is not impaired. Convolutionsof conical barbs 37 are cut out on rod 35 such that edges 39 of barbs 37will engage the fibers of the staged composite laminate upon insertionthereby providing an anchoring effect. Head 41 of barbed quill 33 isgenerally flat to provide a driving surface. Such a head is normallyneeded unless the laminate is thick (where there would be more anchoringto prevent the quills from being pulled from the soft laminate). FIG. 3illustrates another type of quill generally indicated by 43 which wouldnormally be used for a laminate which would be relatively thicker thanthe type quill 33 is best suited for. Thus, quill 43 has a rod 47 havinga portion 48 adjacent head 51 of constant section. This prevents thethickness of rod 47 near head 51 from becoming unacceptably large due tothe increasing taper from point 53.

The material used for the quills should be one that is compatible withcomposite material. Normally this would be a metallic material.Applicant has found that titanium (or an alloy thereof) is particularlysuitable due to its high strength to weight ratio.

With reference again to FIG. 1, a row of barbed metallic quills such asillustrated in FIG. 2 or 3 is imbedded in each of fillet portion 28 ofsheets 20 and 22, through filler material 30 and into cover 18. Quillsaccording to the present invention can be inserted into the compositestructure in the staged condition very quickly, such as by impact hammeror optimally by ultrasonic gun. Suitable ultrasonic guns aremanufactured by the Sonobond Corporation located in Westchester, Pa.After placing quills such as the type 33 or 43 into contact withcomposite sheets 20 and 22, the ultrasonic gun causes such quills tovibrate while forcing them into the composite sheets. This vibrationheats up the quills and in so doing softens the staged epoxy resin whichaids parting of the composite fibers of sheets 20 and 22 by making themmore pliable. In combination with the pointed and tapered shape of thequills, this results in minimal fiber damage (which otherwise wouldimpair structural strength). Quills 12 are oriented at an angle (whichis preferably about 45°) with respect to cover 18. Respective rows ofquills 14 can also be used to supplement fastening of portion 24 ofsheets 20 and 22 to panel 18. Quills 14 are preferably orthogonallydisposed to cover 18. Spacing of quills 12 and 14 along their respectiverows is a matter of design choice. The length of quills 12 and 14 shouldbe such that there is no protrusion below cover 18.

FIG. 4 illustrates a complex composite structure generally indicated at54 where rows of barbed titanium quills 56 are used in joining theassembly in the staged condition. The quills 56, like quills 12 and 14of FIG. 1, allow for lay-up and co-curing of the composite structure asopposed to curing of each of the individual parts prior to lay-up andcuring of the entire assembly. With Applicant's invention, subsequent tolay-up, the individual parts and structure as a whole are co-cured inone operation. This allows large structures, such as a wing box which isillustrated in FIG. 4, to be co-cured. It further saves time and expenseof multiple curing operations and difficulty in joining of rigid curedparts. The barbed quills also add strength to the structural joints asmore fully described hereinafter.

Curing of the assembly, such as 54, is accomplished by thermosetting onthe application of heat and pressure to the composite structure.Typically, when curing an assembly such as that shown in FIG. 4 which isof graphite epoxy material, the assembly would be placed in an autoclaveor oven and heated to a temperature of about 350° F for about one hourwhereupon the panels of the structure become tough and have a permanentshape retention and the assembly itself becomes permanently joined. Thequills which are infixed in the structure become even more anchored inthe structure by virtue of the resin locking around the barbs of thequills during curing.

With reference again to FIG. 1, under normal conditions, without theinfixing of the quills of the present invention, a tension load appliedto panel 18 would, at undesirably low levels, cause a peeling of web 16from panel 18 at fillet areas 28. However, by virtue of quills 12, whichadd tensile strength due to the rod of the quills and especially due tothe barbs which anchor the quill in place by interaction with thecomposite fibers and by virtue of the cured resin which locks around thebarbs, the joint is substantially strengthened and resistance to peelincreased. Optionally, quills 14 can also be provided to furtherstrengthen the joining of web 16 to panel 18 in the same manner. It hasbeen found that a row arrangement of quills 12, such as illustrated inFIG. 1, is a particularly effective arrangement for reinforcingstructure 10. This is basically due to the strengthening of the joiningof the web 16 at its tangent point connection to filler material 30 (orload coupler 60--see FIG. 6) where peel forces are greatest.

FIG. 5 illustrates a load coupler or reinforcement member generallyindicated at 60. Reinforcement member 60 is normally formed as amultilayer laminate by blanking the desired shape from sheets ofcomposite material. The layers or sheets of the laminate are of uniformsize and shape and are stacked to the desired length to formreinforcement member 60. The layers are bonded together by means of athermosetting resin which is contained in the material itself whenacquired in preimpregnated form.

Consideration must be given to the fiber orientation in stacking thelayers of the laminate and it is important that the fibers of thelaminate run in at least two directions. This is illustrated on facesheet 62 of reinforcement member 60 where longitudinal fibers 63 run inthe direction of arrow 64 and transverse fibers 65 in the direction ofarrows 66. For most embodiments, the longitudinal and transverse fiberswill optimally be orthogonally oriented. Normally, this will take theform of a 0°/90° orientation where the 0° axis is parallel to the flatbottom 67 of base 68 of member 60. Thus, the transverse fibers are 0°fibers and the longitudinal fibers are 90° fibers. It should of coursebe understood that other orientations can be used, such as -45°/45°,0°/85°, etc., depending upon the particular geometry of the reinforcingmember 60 and/or the joint to be reinforced.

As can be seen in FIG. 5, reinforcement member (load coupler) 60 isgenerally wedge-shaped. In this regard, it is meant that the structurehas an enlarged base with sides tapering to a thin edge. Member 60 isgenerally defined by a shank 70 and a base 68. Shank 70 has twolongitudinal sides 72 and 74 which converge or taper to a thin upperedge 76. While the interface between shank 70 and base 68 is notspecifically delineated in that each layer of the laminate is preferablyintegral, such can be approximated by considering the transition at afillet area 78 on each side of member 60. Thus, base 68 extendsarcuately outward from shank 70. By virtue of this diverging arcuatecontour and flat bottom 67 of base 68, it is seen that the base 68 istapered as it extends from shank 70.

It can now be seen that a flatwise tension load on the bottom face 67 ofbase 68 through longitudinal fibers 63 and the connected transversefibers 65. Since the great majority of fibers terminate at thelongitudinal sides 72 and 74 of shank 70, the bulk of a tension force onbottom face 67 is transferred to the longitudinal side 72 and 74 alongfillet areas 78.

FIG. 6 illustrates a reinforced composite structure generally indicatedat 80 which utilizes a combination of the load coupler 60 and barbedquills 82 and 84. Such a combination substantially strengthens thestructural joint generally indicated at 86 which is defined by opposedsheets 88 and 90 making up a web generally indicated at 92 and compositepanel or skin 94. Normally web 92 would be joined to upper panel or sparcap 96. A portion 100 of sheets 88 and 90 is positioned parallel andadjacent to panel 94. A second portion 102 of sheets 88 and 90 extendstransversely to panel 94. In the embodiment illustrated in FIG. 6,second portion 102 is approximately orthogonally disposed to panel 94.Sheets 88 and 90 also have a fillet portion 104 between first portion100 and second portion 102. Load coupler 60 is positioned between sheets88 and 90 of web 92 and panel 94 such that the bottom face 67 of coupler60 rests directly on (or is imbedded in) panel 94. As can be seen inFIGS. 5 and 6, the longitudinal fibers of reinforcement member 60 areparallel to the center line (not shown) of web 92 and transverse fibersare parallel to panel 94. It has been found that this is normally theoptimal configuration of such fibers. Thus, a flatwise tension load onpanel 60 is distributed through member 60 to sides 72 and 74 where thereis a double shear bond joint to web 92. This effectively bypasses thepeel prone fillet areas 104 because the great majority of fibers ofmember 60 terminate above fillet areas 104.

Barbed quills 82 are placed in the fillet areas 104 through member 60and into panel 94 as set forth previously. This holds the stagedassembly together to enable co-curing as previously described and alsostrengthens structural joint 80. Barbed quills 84 may also be used foradditional strengthening.

Thus it is apparent that there has been provided, in accordance with theinvention, a reinforced composite structural joint and method offabrication thereof that fully satisfies the objectives, aims, andadvantages set forth above. While the invention has been described inconjunction with specific embodiments thereof, it is evident that manyalternatives, modifications, and variations will be apparent to thoseskilled in the art in light of the foregoing description. Accordingly,it is intended to embrace all such alternatives, modifications, andvariations that fall within the spirit and scope of the appended claims.

What is claimed is:
 1. A reinforced composite structure comprising:astaged composite panel, said panel having a plurality of fiber compositeplies, said plies being adhesively bonded together; a staged compositeweb, said web having at least one sheet of fiber composite material,said sheet having at least one layer, said sheet having three portions,a first portion positioned parallel to said panel, a second portionpositioned transversely to said panel, and a fillet portion between saidfirst and second portions; and a plurality of quills infixed in saidfillet portion and said panel, said quills having barbs thereon, saidbarbs engaging the fibers of said panel and said web, said quillsjoining said fillet portion to said panel whereby said panel and saidweb can be co-cured as assembled.
 2. The reinforced composite structureof claim 1 wherein said quills have a head and a rod, said barbs areprovided on said rod, said rod is positioned approximately perpendicularto said head, said rod having a pointed distal end, and said quills arepositioned such that the structure is reinforced when cured.
 3. Thereinforced composite structure of claim 2 wherein said quills aremetallic and said rod has at least a portion thereof tapered to saiddistal end.
 4. The reinforced composite structure of claim 3 whereinsaid quills are of titanium, and said quills are positionedsubstantially in a row along said fillet portion.
 5. The reinforcedcomposite structure of claim 1 also including a composite reinforcementmember, said member comprising a generally wedge-shaped laminate, saidlaminate being defined by a shank and a base, said shank adjoining saidbase, said shank having two longitudinal sides, said base extendingarcuately outward from said shank, said base being tapered in thicknessas it extends from said shank, said laminate having a plurality of fibercomposite sheets of uniform size and shape, said sheets of said laminatebeing adhesively bonded together, said sheets of said laminate beingarranged such that the fibers thereof substantially distribute a tensionload on said base as a shear load along said longitudinal sides of saidshank, said reinforcement member being bonded to said web and saidpanel, said base adjoining said panel and said fillet portion of saidweb, and said shank adjoining said second portion of said web, saidquills being infixed in said reinforcement member.
 6. The reinforcedcomposite structure of claim 2 also including a composite reinforcementmember, said member comprising a generally wedge-shaped laminate, saidlaminate being defined by a shank and a base, said shank adjoining saidbase, said shank having two longitudinal sides, said base extendingarcuately outward from said shank, said base being tapered in thicknessas it extends from said shank, said laminate having a plurality of fibercomposite sheets of uniform size and shape, said sheets of said laminatebeing adhesively bonded together, said sheets of said laminate beingarranged such that the fibers thereof substantially distribute a tensionload on said base as a shear load along said longitudinal sides of saidshank, said reinforcement member being bonded to said web and saidpanel, said base adjoining said panel and said fillet portion of saidweb, and said shank adjoining said second portion of said web, saidquills being infixed in said reinforcement member.
 7. A reinforcedcomposite structure comprising:a composite panel, said panel having aplurality of fiber composite plies, said plies being bonded together; acomposite web, said web having at least one sheet of fiber compositematerial, said sheet having at least one layer, said sheet having threeportions, a first portion positioned parallel to said panel, said firstportion being bonded to said panel, a second portion positionedtransversely to said panel, and a fillet portion between said first andsecond portion; a composite reinforcement member, said member comprisinga generally wedge-shaped laminate, said laminate being defined by ashank and a base, said shank adjoining said base, said shank having twolongitudinal sides, said base extending arcuately outward from saidshank, said base being tapered in thickness as it extends from saidshank, said laminate having a plurality of fiber composite sheets ofuniform size and shape, said sheets of said laminate being adhesivelybonded together, said sheets of said laminate being arranged such thatthe fibers thereof substantially distribute a tension load on said baseas a shear load along said longitudinal sides of said shank, saidreinforcement member being bonded to said web and said panel, said baseadjoining said panel and said fillet portion of said web, said shankadjoining said second portion of said web; and a plurality of quillsinfixed in said panel, said reinforcement member, and said filletportion of said web.
 8. The reinforced composite structure of claim 7wherein said quills have a head and a rod, said barbs are provided onsaid rod, said rod is positioned approximately perpendicular to saidhead, said rod having a pointed distal end, and said quills arepositioned to reinforce the structure.
 9. The reinforced compositestructure of claim 8 wherein said quills are metallic, and said shankhas at least a portion thereof tapered to said distal end.
 10. Thereinforced composite structure of claim 9 wherein said quills are oftitanium, and said quills are positioned substantially in a row alongsaid fillet portion.
 11. A method of forming a composite structure whichcomprises:providing a composite panel and a composite web, said panelhaving a plurality of fiber composite plies, said plies being bondedtogether, said web having at least one sheet of fiber compositematerial, said sheet having at least one layer, said sheet having threeportions, a first portion and a second portion which are divided by afillet portion; staging said panel and said web; positioning said panelrelative to said web such that said first portion is positioned parallelto said panel and said second portion is positioned transversely to saidpanel; infixing a plurality of quills in said fillet portion and saidpanel while said web and said panel are in a staged condition such thatsaid fillet portion is joined to said panel, said quills having barbsthereon, said barbs engaging the fibers of said panel and said web; andco-curing said web and said panel as joined.
 12. The method of claim 11wherein said infixing of said quills is such that said quills arepositioned to reinforce said structure when cured.
 13. The method ofclaim 12 wherein said quills have a head and a rod, said barbs areprovided on said rod, said rod is positioned approximately perpendicularto said head, and said rod has a pointed distal end.
 14. The method ofclaim 13 wherein said quills are metallic and said rod has at least aportion thereof tapered to said distal end.
 15. The method of claim 14wherein said quills are of titanium, and said quills are positioned in arow along said fillet portion.
 16. The method of claim 11 wherein saidinfixing a plurality of quills is with an ultrasonic gun.
 17. The methodof claim 11 wherein said panel and said web are bonded together duringsaid co-curing step.